Ring laser gyroscopes have been in use in industries such as the aerospace industry to measure change of position of a platform on which they are mounted, for example, on aircraft. Typically, such devices operate by generating two laser beams travelling in opposite directions within a closed-loop cavity. One example of such a device has a three-sided path which is defined by a laser-light generating gas-containing chamber, with mirrors located at the corners of each of the paths to reflect laser beams travelling therein in a new direction along a different path of a cavity within the device.
The frequency of the laser beams is very high, approximately, for example, 500 Terahertz for a helium-neon laser. This frequency is much higher than the response of, for example, typical light detecting diodes, frequently referred to as photodiodes, and their associated electronics employed in such gyroscopes. Consequently, only the average intensity of the laser beams appears at the output of a photodetector and its associated electronics, and it appears as a DC (direct current) signal.
If the gyroscope is at rest, then the frequencies of the two laser beams are identical. But if there is a large angular input rate applied to the input axis of the gyroscope, the two laser beams assume different frequencies, with the difference in the frequencies being approximately proportional to the applied input rate. If the two laser beams are then combined (interfered) with each other, a beat frequency equal to the frequency difference between the laser beams results. In a typical ring laser gyroscope, this beat frequency is within the bandwidth capabilities of the photodetectors and their associated electronics, and appears as an AC (alternating current) component riding on top of the DC signal that is due to the average intensity of the laser beams. In a ring laser gyroscope, the two laser beams are purposely interfered with each other to generate this beat frequency, which, for example, is detected with photodiodes to provide a signal on which the frequency of the AC component is proportional to input rate.
In order to determine the direction of rotational movement of the gyroscope or object upon which it is mounted, i.e., rotation in either clockwise or counterclockwise direction, the two diodes are employed to detect a phase shift at 90.degree. from each other. In accordance with the output, in addition to the detected change in interference, the measurement from the diodes is then used to determine whether the direction of movement is in a clockwise or counterclockwise rotation.
Those of ordinary skill in the art recognize that, if there is any scattering within the lasing cavity of a ring laser gyroscope so that a portion of one of the beams couples into the other beam, there is a tendency for the phases of the two beams to pull towards each other. This pulling effect is most apparent when the frequencies and phases of the two laser beams travelling in opposite directions within the same cavity are close to each other, such as when the input rate approaches zero. This pulling of the frequencies and the phases of the two beams towards each other, frequently referred to as lock-in, results in an error in the output of the gyroscope.
One means available in the prior art to reduce this pulling between the beams is to modulate the ring laser gyroscope about its input axis in a periodic fashion, this modulation frequently being referred to as dither, as discussed in U.S. Pat. No. 3,373,650 issued Mar. 19, 1968 to J. E. Killpatrick, and assigned to the same assignee as that of the present invention. This patent teaches that this dither motion reduces this lock-in effect the majority of the time. A further development in reducing such pulling between laser beams is disclosed in U.S. Pat. No. 3,467,472, also issued to J. E. Killpatrick and assigned to the same assignee as that of the present invention, which teaches that the error due to lock-in can be further reduced by randomizing the oscillation or dithering of the beams so that the small errors in the extremities of the oscillation are no longer cumulative. However, those of ordinary skill in the art recognize that, even with the significant benefits derived from the utilization of the teachings of these two mentioned techniques of U.S. Pat. Nos. 3,373,650 and 3,467,472, there is still a small error in the angular output of the ring laser gyroscope which manifests itself as a drift with the characteristics of a statistical random walk. This angular random walk error, of necessity, affects the accuracy of the measurements being conducted.
U.S. Pat. No. 4,152,071, issued May 1, 1979 to T. J. Podgorski and assigned to the same assignee as the present invention, teaches that random walk can be reduced by shifting the position of the mirrors in the gyroscope so as to change the position of the path that the laser beams traverse within the lasing cavity of the ring laser gyroscope. By shifting the position of some of the mirrors, for example, one mirror can be shifted inwardly while another can be shifted outwardly, it is possible to reduce the random drift or angular random walk of the gyroscope while maintaining the same laser path length. This is accomplished because the new path that the laser beams traverse moves the beams away from any sources of scatter within the lasing cavity.
Though the techniques discussed in U.S. Pat. No. 4,152,071 generally do reduce the random drift of a ring laser gyroscope, those skilled in the art recognize that these techniques have certain limitations. For example, if the input rate applied to the ring laser gyroscope exceeds the peak dither rate, then the discriminant signal, referred to as the single beam signal, which is used to determine whether the gyroscope is operating at a minimum in angular random walk, as discussed in U.S. Pat. No. 4,152,071, is significantly reduced in amplitude so that the techniques discussed in U.S. Pat. No. 4,152,071 are no longer practically useable. In addition, the modulation discussed in U.S. Pat. No. 4,152,071, of necessity periodically moves the path that the laser beams traverse from that which results in minimal random walk. Further, the response of circuits using the techniques discussed in U.S. Pat. No. 4,152,071 is limited by the response of the integrator employed therein.
In accordance with this present invention, the problems of the prior art are either minimized or avoided and it becomes possible to minimize the amount of angular random walk in accordance with a very precise method and specific arrangement of components which, in addition, provides great flexibility to the method and device used to reduce such angular random walk.